Today, the determination of the steering angle is very important in the field of vehicle dynamics control, in which it is possible, for example, to use steering angle sensors. Electronic Stability Control (ESC) is the term used for systems designed to improve a vehicle's handling, particularly at the limits where the driver might lose some control of the vehicle. Stability control systems require a steering wheel angle (SWA) sensor to assess driver intent. A stabilization device can be an electronic stability program (ESC unit) while a servo-assisted steering device can be, for example, an EPAS (Electric Power Assisted Steering). However, even in the field of what is referred to as steer by wire, i.e. the electronic transmission of steering signals to the steered wheels, steering angle sensors are used.
In many cases, the steering angle sensors typically transmit the angular position of the steering wheel which has a close connection to the position of the steered wheels and thus to the direction of travel of the motor vehicle. For example, in an ESC unit, it compares the driver's intended direction in steering and braking inputs, to the vehicle's response, by monitoring lateral acceleration, rotation (yaw) and individual wheel speeds. An ESC system then brakes individual front or rear wheels and may reduce excess engine power as needed, to help correct understeer plowing or oversteer. Therefore, the steering angle is transmitted in order to determine the direction in which the driver intends to travel. By doing this, the steering angle sensor converts the steering movements of the driver into electrical signals for the ESC unit or the ESC system.
It is a well known method to implement a steering angle sensor in what is referred to as an absolute steering angle sensor. An absolute steering angle sensor transmits the absolute angular position of the steering wheel or of the steering column of a motor vehicle to an evaluation unit, for example to the ESC unit. The absolute position of the steering wheel is transmitted to the ESC unit, using a shaft position encoder. In addition, the reasonableness of the value can also be checked with further input signals such as, for example, the yaw rate, lateral acceleration and/or wheel speed. However, this possibility is very costly.
What are referred to as relative steering angle sensors are also well known in the art and used as a further cost-effective alternative. Relative steering angle sensors transmit only relative steering angle changes from any desired initial position when the steering wheel or the steering column is rotated. The evaluation unit, for example the ESC unit, calculates the absolute position of the steering wheel on the basis of the changes in the steering angle or the relative steering angle signals and additional information which are obtained by means of further sensors. Additional information which can be picked up are, for example the yaw rate, lateral acceleration and/or wheel speed. The duration and quality of the determination of the absolute steering wheel position is, however, dependent on the driving style and it is necessary to check the reasonableness of all the input signals with respect to one another, which can take a relatively long time under certain circumstances. During this time, the ESC is available only to a limited degree.
EPAS systems today are capable of determining the absolute steering angle. The disadvantage of these systems is that the absolute position is not determined using proven ESC algorithms but rather using newly developed algorithms, for which only reduced sensor information is available and which takes significantly longer to determine the absolute steering angle than the proven ESC algorithms. The plausibility checking which is necessary after the vehicle is switched on is also carried out with these new algorithms and is therefore slower and less precise.
What is needed is an improved method for determining the steering wheel angle using simple means to dispense with additional costly relative and absolute steering angle sensors, while at the same time unreasonable and faulty steering wheel angle states can be reliably detected.